1. Field of the Invention
The present invention relates to a method for producing high purity glycolic acid crystals from an aqueous glycolic acid solution. More particularly, the present invention is concerned with a method for producing high purity glycolic acid crystals from an aqueous glycolic acid solution, which comprises the steps of: providing an aqueous glycolic acid solution which contains monomeric glycolic acid and a glycolic acid condensation product, and has a specific water content, a specific calculated monomeric glycolic acid weight ratio (ratio of the total weight of the monomeric glycolic acid and the component monomeric glycolic acid of the glycolic acid condensation product to the weight of the aqueous solution) and a specific monomeric glycolic acid content; depositing glycolic acid crystals from the aqueous glycolic acid solution; and separating the deposited glycolic acid crystals from the aqueous glycolic acid solution. By the method of the present invention, it becomes possible to produce high purity glycolic acid crystals easily and in high yield on a commercial scale, which high purity glycolic acid crystals are essential for producing a glycolic acid polymer having a high molecular weight.
2. Prior Art
Conventionally, glycolic acid has been used as an important component of cosmetics, a hair dying agent, a shampoo, detergents (e.g., a detergent for domestic use and a detergent for industrial use), a metal treating agent, a tanning agent and the like. In recent years, glycolic acid is also widely used as a raw material for various chemical products, a raw material for various synthetic resins and the like.
It is required that glycolic acid for use as the above-mentioned raw materials have high purity. The reason for this is explained below, taking as an example the case where glycolic acid is used as a raw material for a synthetic resin, wherein the glycolic acid is required to have an extremely high purity.
In general, for obtaining a polyester resin comprised mainly of glycolic acid monomer units, which exhibits a mechanical strength sufficient for general use, it is desired that the polyester resin have a weight average molecular weight of 150,000 or more (see, for example, Unexamined Japanese Patent Application Laid-open Specification No. Hei 11-130847 (corresponding to WO99/19378)). When such a polyester resin is produced by polycondensation of a raw material mixture containing glycolic acid as a main component thereof, care must be taken as to the following points. Glycolic acid is a self-condensing compound having, in a molecule thereof, one carboxyl group and one alcoholic hydroxyl group. Therefore, when the raw material mixture contains, as an impurity, a carboxylic acid having no alcoholic hydroxyl group or an alcohol having no carboxyl group, such an impurity inevitably reacts with glycolic acid and/or a polycondensation product of glycolic acid, thereby terminating the growth of the polymer chain. As a result, a polyester resin having a high molecular weight cannot be obtained. Further, when the raw material mixture contains a salt as an impurity, the mechanical property of the polyester resin obtained is markedly lowered. Accordingly, when the production of a polyester resin is performed by poly-condensation of a raw material mixture containing glycolic acid as a main component thereof, it is necessary that the glycolic acid have a high purity. In general, the glycolic acid is required to have a purity as high as 99.9% by weight or more.
As conventional methods for producing glycolic acid on a commercial scale, there can be mentioned:    (a) a method in which glycolic acid is produced by reacting formaldehyde, carbon monoxide and water in the presence of an acid catalyst; and    (b) a method in which chloroacetic acid is saponified.
In each of the above-mentioned methods (a) and (b), glycolic acid is obtained in the form of an aqueous solution thereof. However, in each of these methods, a carboxylic acid and an alcohol are by-produced in large amounts, and a salt is by-produced in an amount such that the mechanical properties of a resin produced from the glycolic acid become markedly low. Therefore, for obtaining a high purity glycolic acid which can be used as a raw material for producing a resin having a high molecular weight, it is necessary that the aqueous glycolic acid solution be subjected to purification.
As an example of a generally employed purification method, there can be mentioned a distillation method. When the purification of glycolic acid is performed by distillation method, a distillate containing glycolic acid is obtained, and the obtained distillate is cooled and solidified to thereby obtain high purity glycolic acid crystals. However, it is difficult to obtain high purity glycolic acid crystals by the distillation of the above-mentioned aqueous solution of glycolic acid. The reason for this is as follows. Glycolic acid has low volatility. Further, glycolic acid is susceptible to a polycondensation reaction under the distillation conditions, so that a condensation product which is difficult to distill is formed. Therefore, it is difficult to obtain high purity glycolic acid crystals by the distillation method.
For these reasons, there have been proposed methods (other than the distillation method) for producing high purity glycolic acid crystals from an aqueous glycolic acid solution.
For example, Unexamined Japanese Patent Application Laid-open Specification No. Hei 8-268955 (corresponding to EP 733616 and BR 9601063) describes a method in which water is removed from an aqueous glycolic acid solution under specific conditions so as to form a molten product containing glycolic acid, and a crystallization agent is added to the formed molten product, followed by cooling to produce glycolic acid crystals. However, when it is intended to produce high purity glycolic acid crystals by this method, it is necessary to purify, in advance, the aqueous glycolic acid solution to a very high degree by a method, such as electrodialysis, solvent extraction or the like. Therefore, when it is intended to produce high purity glycolic acid crystals by this method, many complicated steps become necessary and, thus, the entire process becomes very cumbersome.
As another method for producing glycolic acid crystals from an aqueous glycolic acid solution, Japanese Patent Application Prior-to-Examination Publication (Tokuhyo) No. Hei 6-501268 (corresponding to WO92/05138 and EP 552255) describes a method in which an aqueous glycolic acid solution containing about 62.4% by weight of monomeric glycolic acid, about 8.8% by weight of dimeric glycolic acid, about 2.2% by weight of diglycolic acid, about 2.2% by weight of methoxyacetic acid, and about 0.24% by weight of formic acid is used as a starting material, and is cooled to −25 to 10° C., followed by addition of high purity glycolic acid crystals, as seed crystals, in an amount sufficient to induce the crystallization of glycolic acid in the solution, thereby obtaining glycolic acid crystals. In this patent document, it is described that high purity glycolic acid crystals can be obtained depending on the conditions for crystallization of glycolic acid. However, when it is intended to obtain high purity glycolic acid crystals in high yield by the method described in the patent document, a very long period of time is needed for crystallization of glycolic acid and, thus, the method becomes economically disadvantageous.
Further, the above-mentioned patent document also describes a method in which, in an attempt to improve the yield of the glycolic acid crystals, a mother liquor after the separation of glycolic acid crystals is concentrated, and the resultant concentrated mother liquor is subjected to a further crystallization. However, this method has a problem in that the purity of glycolic acid crystals recovered after the further crystallization is markedly low.
Further, there is also known a method for producing high purity glycolic acid crystals, which, however, is difficult to practice on a commercial scale. Specifically, in this method, crude glycolic acid crystals are produced and dissolved in an organic solvent, such as acetone, and the resultant solution is allowed to stand while cooling the solution to −10° C., thereby obtaining high purity glycolic acid crystals. Further, Unexamined Japanese Patent Application Laid-open Specification No. Hei 5-92102 describes a method in which crude glycolic acid crystals are dissolved in an organic solvent, such as acetone, to obtain a solution, followed by mixing thereof with high pressure gas, thereby obtaining high purity glycolic acid crystals. However, these methods are difficult to practice on a commercial scale for the following reasons. When it is intended to practice any of the above-mentioned methods on a commercial scale, the use of a large amount of a harmful organic solvent is necessary and the whole amount of the solvent used must be removed or recovered by distillation. Therefore, from the viewpoint of protection of environment and economy, it is impossible to practice the above-mentioned methods on a commercial scale.
Thus, by the conventional methods, it has been impossible to produce high purity glycolic acid crystals from an aqueous glycolic acid solution easily and in high yield on a commercial scale.